The present invention relates to a synchronization signal generating unit of an image forming apparatus, and more particularly relates to a synchronization signal generating unit used for an image forming apparatus in which a plurality of light beams are simultaneously scanned so that a plurality of lines can be simultaneously recorded, the synchronization signal generating unit characterized in that the deviations of the plurality of light beams are detected in the scanning direction, and a synchronization signal corresponding to each light beam is generated in accordance with the result of the detection.
In an image forming apparatus, image information is recorded on a recording medium in such a manner that a laser beam (light beam) modulated in accordance with an image signal is deflected by a rotary polygonal mirror and scanned on the recording medium. In this image forming apparatus, in order to increase the recording speed, a plurality of laser beams are used and a plurality of lines are simultaneously recorded.
When a plurality of laser beams are simultaneously subjected to scanning in the manner described above, deviation tends to occur between the laser beams, so that the image recording position of each line deviates and stable image formation of high fidelity can not be made.
In order to solve the above problems, various apparatus are proposed in Japanese Patent Publication Open to Public Inspection Nos. 188713/1990 and 39669/1982. According to the apparatus, the operation is carried out as follows. The scanning position of each laser beam is detected, and a synchronization signal (horizontal synchronization signal) is generated for each beam. Therefore, even if the beam position deviates with respect to the scanning direction because of the environmental condition or aging change, image formation of high fidelity can be made.
For example, in the image forming apparatus disclosed in Japanese Patent Publication Open to Public Inspection No. 188713/1990, a synchronization signal corresponding to each beam is generated using one sensor when a plurality of beams are successively detected in the time series through a slit member.
In the synchronization signal generating unit disclosed in Japanese Patent Publication Open to Public Inspection No. 39669/1982, a synchronization signal corresponding to each beam is generated in accordance with a change in the output of a beam detecting means, utilizing that the output of the beam detecting means is changed stepwise by the number of incident beams.
The correlation between a current flowing in a semiconductor in the normal direction and an optical output corresponding to the current deviates among the individual semiconductor lasers. Further, it greatly deviates due to the environmental temperature. By the deviation of the optical output in a plurality of semiconductor lasers, the laser beam diameter deviates.
It is necessary that a synchronization signal is generated in accordance with the deviation of a beam center. However, when the beam diameter deviates, even if the position of the beam center is the same, the time of beam detection is quickened or delayed, so that an error is caused in the generation time of a synchronization signal.
The present invention has been achieved to solve the above problems. It is a first object of the present invention to provide a synchronization signal generating unit used for an image forming apparatus in which a plurality of light beams are simultaneously scanned for recording an image, the synchronization signal generating unit characterized in that the positional deviation of the light beam in the scanning direction is accurately detected, so that the synchronization signal corresponding to each light beam can be accurately generated.
In this connection, according to the construction disclosed in Japanese Patent Publication Open to Public Inspection No. 39669/1982, a horizontal synchronization signal of each laser beam is generated in accordance with a stepwise change in the output of the optical detector. Therefore, the time to generate a synchronization signal is changed being affected by the characteristics of the detection output of the optical detector. For this reason, the output characteristics of the optical detector must be highly accurate and stable.
According to the construction disclosed in Japanese Patent Publication Open to Public Inspection No. 188713/1990, unless the opening of a slit member is at a predetermined position relative to the primary scanning direction, deviation of the scanning position of the laser beam in the primary scanning direction is erroneously detected. Further, it is difficult to detect from the beam detection signal of the optical detector whether or not the aforementioned relative positional relation is correct. Therefore, it is necessary that the assembly is previously conducted with high accuracy and the synchronization signal is generated under the condition that the aforementioned predetermined positional relation is maintained.
The present invention has been achieved in view of the above problems. The second object of the present invention is to provide a synchronization signal generating unit characterized in that: a horizontal synchronization signal can be generated only in accordance with the result of detection of the light beam; and the accuracy of the horizontal synchronization signal generating time can be maintained high corresponding to the positional deviation of the construction for detecting the light beam.